The present invention relates to a shift control system for an automatic transmission.
The automatic transmission is shiftable in speed ratio by selecting a predetermined speed by selective hydraulic actuation of friction elements such as a clutch and a brake to change the friction elements to be operated.
Accordingly, some types of shifting necessitate redeployment of the friction elements such that one friction element or high clutch is engaged whereas the other friction element or band brake is released, as is seen during a 2-3 shift of the automatic transmission described in a Maintenance Manual for NISSAN FE4F02A type Automatic Trans-axle, published by NISSAN MOTOR CO. LTD., in March, 1988. Such shifting by redeployment of the friction elements as represented by the automatic transmission described in the above-mentioned document is ensured by a structure as shown in FIG. 6.
Referring to FIG. 6, H/C designates a high clutch, B/B designates a band brake, 1 designates 1-2 shift valve, 2 designates a 2-3 shift valve, and L/C designates a low clutch. With the shift valves 1, 2 turned off as illustrated in FIG. 6, the high clutch H/C and the band brake B/B are both released, and only the low clutch L/C is engaged, selecting first speed. In that state, if the 1-2 shift valve 1 is turned on, the band brake B/B is supplied with line pressure P.sub.L as engagement pressure P.sub.B/A to produce engagement thereof. This additional actuation of the band brake B/B causes the automatic transmission to have an upshift change of speed from first to second speed.
In that state, if the 2-3 shift valve 2 is also turned on, line pressure P.sub.L is supplied to the high clutch H/C as engagement pressure P.sub.H/C, and to the band brake B/B as release pressure P.sub.B/R. Pressure P.sub.H/C engages the high clutch H/C, and pressure P.sub.B/R releases the band brake B/B due to a large pressure acting area thereof in spite of the existence of engagement pressure P.sub.B/A. Thus, redeployment of the friction elements is accomplished such that the high clutch H/C is engaged whereas the band brake B/B is released, which causes the automatic transmission to have an upshift change of speed from second to third speed.
With the known control circuit for redeployment of the friction elements, assuming that an engagement pressure acting area of the band brake B/B is A.sub.A, a release pressure acting area thereof is A.sub.R, and a force of a return spring is F.sub.S, the balance of forces of the band brake B/B is expressed by the following equation: EQU P.sub.B/A .multidot. A.sub.A = P.sub.B/R .multidot. A.sub.R + F.sub.S
Accordingly, the release pressure Phd B/R of the band brake B/B is expressed: ##EQU1##
It will be understood that since the band brake B/B is released when P.sub.B/R, i.e., the engagement pressure P.sub.H/C of the high clutch H/C, is equal to a value given by the equation (1), an engagement timing of the high clutch H/C and a release timing of the band brake B/B cannot be controlled individually. As a result, the two timings cannot be adjusted properly or at desired settings in accordance with the cruising condition and specification of a vehicle.
It is, therefore, an object of the present invention to provide a shift control system for an automatic transmission which can properly adjust an engagement timing of one friction element and a release timing of the other friction element without producing interlock of the automatic transmission.